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Title: Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment

Abstract

Measurements of particle and heat transport have been made in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technology 27, 273 (1995)]. Experimental differences in the density and temperature profiles are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The electron temperature is higher in the QHS configuration, due to a reduction in electron thermal diffusivity that is comparable to the neoclassical prediction. The density profile in plasmas with the symmetry broken is measured to be hollow, while in QHS plasmas the profile is centrally peaked. Calculations of the radial particle flux using the DEGAS code [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)] show that the hollow profile observed with the symmetry broken is due to neoclassical thermodiffusion. Thermodiffusion is reduced in the QHS configuration, resulting in a peaked density profile.

Authors:
; ; ; ; ; ;  [1]
  1. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
Publication Date:
OSTI Identifier:
20975046
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physics of Plasmas; Journal Volume: 14; Journal Issue: 5; Other Information: DOI: 10.1063/1.2709862; (c) 2007 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
70 PLASMA PHYSICS AND FUSION TECHNOLOGY; CONFIGURATION; ELECTRON TEMPERATURE; ELECTRONS; HEAT TRANSFER; ION TEMPERATURE; MAGNETIC FIELDS; NEOCLASSICAL TRANSPORT THEORY; PLASMA; PLASMA CONFINEMENT; PLASMA DENSITY; STELLARATORS; THERMAL DIFFUSION; THERMAL DIFFUSIVITY

Citation Formats

Canik, J. M., Anderson, D. T., Anderson, F. S. B., Clark, C., Likin, K. M., Talmadge, J. N., and Zhai, K. Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment. United States: N. p., 2007. Web. doi:10.1063/1.2709862.
Canik, J. M., Anderson, D. T., Anderson, F. S. B., Clark, C., Likin, K. M., Talmadge, J. N., & Zhai, K. Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment. United States. doi:10.1063/1.2709862.
Canik, J. M., Anderson, D. T., Anderson, F. S. B., Clark, C., Likin, K. M., Talmadge, J. N., and Zhai, K. Tue . "Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment". United States. doi:10.1063/1.2709862.
@article{osti_20975046,
title = {Reduced particle and heat transport with quasisymmetry in the Helically Symmetric Experiment},
author = {Canik, J. M. and Anderson, D. T. and Anderson, F. S. B. and Clark, C. and Likin, K. M. and Talmadge, J. N. and Zhai, K.},
abstractNote = {Measurements of particle and heat transport have been made in the Helically Symmetric Experiment [F. S. B. Anderson et al., Fusion Technology 27, 273 (1995)]. Experimental differences in the density and temperature profiles are reported between plasmas produced in a quasihelically symmetric (QHS) magnetic field and a configuration with the symmetry broken. The electron temperature is higher in the QHS configuration, due to a reduction in electron thermal diffusivity that is comparable to the neoclassical prediction. The density profile in plasmas with the symmetry broken is measured to be hollow, while in QHS plasmas the profile is centrally peaked. Calculations of the radial particle flux using the DEGAS code [D. Heifetz et al., J. Comput. Phys. 46, 309 (1982)] show that the hollow profile observed with the symmetry broken is due to neoclassical thermodiffusion. Thermodiffusion is reduced in the QHS configuration, resulting in a peaked density profile.},
doi = {10.1063/1.2709862},
journal = {Physics of Plasmas},
number = 5,
volume = 14,
place = {United States},
year = {Tue May 15 00:00:00 EDT 2007},
month = {Tue May 15 00:00:00 EDT 2007}
}
  • Electron cyclotron heated plasmas in the Helically Symmetric Experiment (HSX) feature strongly peaked electron temperature profiles; central temperatures are 2.5 keV with 100 kW injected power. These measurements, coupled with neoclassical predictions of large 'electron root' radial electric fields with strong radial shear, are evidence of a neoclassically driven thermal transport barrier. Neoclassical transport quantities are calculated using the PENTA code [D. A. Spong, Phys. Plasmas 12, 056114 (2005)], in which momentum is conserved and parallel flow is included. Unlike a conventional stellarator, which exhibits strong flow damping in all directions on a flux surface, quasisymmetric stellarators are free tomore » rotate in the direction of symmetry, and the effect of momentum conservation in neoclassical calculations may therefore be significant. Momentum conservation is shown to modify the neoclassical ion flux and ambipolar ion root radial electric fields in the quasisymmetric configuration. The effect is much smaller in a HSX configuration where the symmetry is spoiled. In addition to neoclassical transport, a model of trapped electron mode turbulence is used to calculate the turbulent-driven electron thermal diffusivity. Turbulent transport quenching due to the neoclassically predicted radial electric field profile is needed in predictive transport simulations to reproduce the peaking of the measured electron temperature profile [Guttenfelder et al., Phys. Rev. Lett. 101, 215002 (2008)].« less
  • A new constant of motion for helically symmetric equilibria in the vicinity of the magnetic axis is obtained in the framework of Vlasov theory. In view of this constant of motion the Vlasov theory is compared with drift kinetic and gyrokinetic theories near axis. It turns out that as in the case of axisymmetric equilibria [H. Tasso and G. N. Throumoulopoulos, Phys. Plasmas 18, 064507 (2011)] the Vlasov current density thereon can differ appreciably from the drift kinetic and gyrokinetic current densities. This indicates some limitation on the implications of reduced kinetic theories, in particular, as concerns the physics ofmore » energetic particles in the central region of magnetically confined plasmas.« less
  • HSX is a quasi-helically symmetric (QHS) stellarator currently under construction at the Torsatron-Stellarator Laboratory of the University of Wisconsin-Madison. This device is unique in its magnetic design in that the magnetic field spectrum possesses only a single dominant (helical) component. This design avoids the large direct orbit losses and the low-collisionality neoclassical losses associated with conventional stellarators. The restoration of symmetry to the confining magnetic field makes the neoclassical confinement in this device analogous to an axisymmetric q=1/3 tokamak. The HSX device has been designed with a clear set of primary physics goals: demonstrate the feasibility of construction of amore » QHS device, examine single particle confinement of injected ions with regard to magnetic field symmetry breaking, compare density and temperature profiles in this helically symmetric system to those for axisymmetric tokamaks and conventional stellarators, examine electric fields and plasma rotation with edge biasing in relation to L-H transitions in symmetric versus non-symmetric stellarator systems, investigate QHS effects on 1/v regime electron confinement, and examine how greatly-reduced neoclassical electron thermal conductivity compares to the experimental {chi}{sub e} profile. 3 refs., 4 figs., 1 tab.« less
  • An eight channel electron cyclotron emission imaging (ECEI) diagnostic system is being developed for use on the helically symmetric experiment (HSX) stellarator. The system utilizes a linear Schottky diode mixer/receiver array, coupled with low cost conventional ECE radiometer IF boards under development at U.C. Davis, to measure the second harmonic x-mode radiation from the plasma. The array is fed with a fixed local oscillator, with single sideband operation enabled via a dichroic plate high pass filter whose cutoff frequency equals that of the local oscillator. A multithrow, high speed ({lt}100 ns switching times) microwave switch is utilized to sequentially connectmore » the output from each mixer channel to the broadband IF receiver system, thus making the array capable of generating a high resolution two-dimensional image of the HSX electron temperature profile at rates exceeding 50 kHz. Instrument details of the system will be described. {copyright} {ital 1999 American Institute of Physics.}« less